Record and discriminated unions in C# 9

recordsanddiscriminatedunions.cs

public class Person  
{  
  public initonly string Firstname { get; }  
  public initonly string Lastname { get; }  
};  

enum class ByteOrBool { byte Y; bool B;} 

enum class MixedType  
{  
  Person P;  
  ByteOrBool U;  
}

var bart = new Person()  
{  
  Firstname = ”Bart”;  
  Lastname = “Simpson”;  
};  

var lisa = bart with { Firstname = "Lisa" };
  
 
var unionRecord = new MixedType.P(bart);  
var unionType1 = new MixedType.U(B true);  
var unionType2 = new MixedType.U(Y 0x42); 

Looks great!

it would also be nice if it can infer types:
Person p = new { Firstname = "Homer", Lastname = "Simpson" };

I agree. I think they are at least considering to allow omitting the „new“ keyword.

All the design proposals are here https://github.com/dotnet/csharplang/tree/master/proposals
(I am in no way involved in the design, just wanted to understand the current status and made this gist in the process.)

What happened to primary constructors? That's a lot of code in place of just class Person(string FirstName, string LastName)

Good question. It seems that primary constructors are still part of the overarching records proposal but it looks like there are quite some issues that need to be worked out.

There is also a wild discussion about it here: dotnet/csharplang#2691